126 OEGANIC CONSTITUENTS OF PLANTS. 



The first created plants in all probability 'did so, but in the present 

 day the decaying remains of other plants and of animals are also con- 

 cerned in the support of vegetation. 



Organic Constituents and their Sources. 



Caebon (0) is the most abundant element in plants. It forms 

 from 40 to 50 per cent of all the plants usually cultivated for food. 

 When plants are charred the carbon is left, and as it enters into all 

 the tissues, although the weight of the plants is diminished by the 

 process, their form still remains. When converted into coal (a form 

 of carbon), plants are frequently so much altered by pressure as to 

 lose their structure, but occasionally it can be detected under the 

 microscope. Carbon is insoluble, and therefore cannot be absorbed in 

 its uncombined state. When united with oxygen, however, in the 

 form of carbonic acid, it is readily taken up either in its gaseous state 

 by the leaves, or in combination with water by the roots. The humus 

 or vegetable mould in the soil contains carbon, and in soils of a peaty 

 nature it exists in very large quantity. The carbon in the soil is 

 converted into carbonic acid in order to be made available for the 

 purpose of plant-growth. Carbon has the power of absorbing gases, 

 and in this way, by enabling certain combinations to go on, it assists 

 in the nourishment of plants. In the atmosphere, carbonic acid is 

 always present, averaging about Tnnr part, arising from the respiration 

 of man and animals, combustion, and other processes. A certain 

 atmospheric equilibrium is thus maintained, consequent on the dif- 

 ference between vegetable and animal respiration, the latter giving 

 out carbonic acid, which the former consumes. 



Oxygen (0) enters into the composition of all plants, but never 

 in quantity sufficient to convert all the hydrogen and carbon present 

 in the plant into water and carbonic acid. In the ash of plants, 

 oxygen, next to carbon, is the most abundant constituent. Oxygen in 

 the air amounts to about 20' 9 per cent, and it forms f by weight of 

 water. Combined with various elements it forms a great part of the 

 soil and solid crust of the earth. It is chiefly in its state of combina- 

 tion with hydrogen to form water (H^O) that oxygen is taken up by 

 plants, but also as carbonic acid (CO^) and oxysalts. 



Hydkogen (H) is not found in a free state in nature, and with 

 the exception of coal, it does not enter into the composition of the ■ 

 mineral masses of the globe. It forms i by weight of water, and it 

 is present in the atmosphere in combination with nitrogen. It is also 

 found in the air united with sulphur (S) and carbon, as a product of 

 vegetable decay. It is mostly from the decomposition of water by 

 the combined action of chlorophyll and sunlight that plants obtain 

 their supply of hydrogen. , 



